As known stack molds include a centre mold part, generally the female part of the mold, and two outside male mold parts which open and close relative to the centre mold part. Typically the opening and closing of these stack molds is controlled through a rack and pinion gear arrangement comprising individual racks from the two outside mold parts which are driven off one big pinion gear located at the centre mold part. The racks on the outside mold parts extend across the centre mold part only as far as the outside surface of the opposite outside mold part when the stack mold is closed. It is not desirable to increase the length on the racks beyond this point otherwise they interfere with the outside mounting surfaces for the mold parts which would then require special modifications to accomodate the increased length of the racks.
The required relatively limited length of the racks in combination with a single pinion gear for controlling rack movement results in a relatively limited opening and somewhat poor access to the stack mold which may for example make it awkward to eject the molded pieces even with the stack mold in the fully opened position.